1. Field of the Invention
The present invention relates to a method of and an apparatus for producing a silicon single crystal having an excellent dielectric breakdown strength of the silicon oxide film formed thereon.
2. Description of the Prior Art
It is well known in the art that a silicon single crystal grown by a pulling method commonly called the Czochralski (CZ) method is produced by an apparatus which comprises a main chamber having a side wall and a top wall disposed on an upper end of the side wall and having an open central portion extending upwardly so as to form a neck portion at an upper end of the main chamber, a quartz crucible disposed in the main chamber with a heater and a thermal shield disposed between the quartz crucible and the side wall of the main chamber, and a gate valve, a pull chamber and a crystal pulling mechanism that are disposed above the neck of the main chamber for pulling a growing silicon single crystal from a silicon melt retained in the quartz crucible. It is also known that single crystal wafers (CZ wafers) prepared from the Czochralski-grown silicon single crystal have an oxide film dielectric breakdown strength which is much smaller than that of single crystal wafers (FZ wafers) prepared from a silicon single crystal grown by the float-zone (FZ) method and wafers (epitaxial wafers) with a silicon film grown by epitaxy on a single crystal substrate prepared by the CZ method. However, due to various advantageous features, the CZ wafers are widely used as a material for fabricating electronic devices. In recent years, with an increase in degree of integration of MOS devices, a demand for a MOS device having a highly reliable gate oxide film had increased. Since the reliability of the gate oxide film is influenced very much by the oxide film dielectric breakdown strength, there is a great need for a CZ wafer having an excellent oxide film dielectric breakdown strength.
To meet the foregoing demand, an attempt has been proposed, as disclosed in Japanese Patent Laid-open Publication No. 2-267195. According to the disclosed prior attempt, a CZ wafer having an excellent oxide film dielectric breakdown strength can be produced by controlling the pulling rate (i.e., the crystal growth rate) of a silicon single crystal below 0.8 mm/min.
The prior attempt is, however, not fully satisfactory because the silicon single crystals produced at the pulling rate 0.8 mm/min may have an insufficient oxide film dielectric breakdown strength. In addition, such a low pulling rate which is smaller than the conventional pulling rate of about 1.2 mm/min brings about a substantial reduction in productivity and a substantial increase in manufacturing cost of the silicon single crystal.